# 76mm airframe at 2.5kg burning an I500T-14A...T/W at 21+?

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#### Ralph M Bohm

##### Well-Known Member
Hello all,

I am scratch-building an improved version of my first scratch-built 76-mm diameter airframe and am planning to burn an I500T-14A Blue Thunder in it.

I am getting a thrust-to-weight ratio of more than 21. Does that sound right?

##### Well-Known Member
TRF Supporter
max thrust of about 130 lbs force. Weights 5.5 lbs. So 23 ish G's

#### Steve Shannon

##### Well-Known Member
TRF Supporter
2500 gram rocket + 576 gram motor + parachute? Or did you already include the mass of the parachute in the rocket weight?
So, 3.076 kilogram rocket mass. Multiply by 9.81 to get the weight of the rocket in Newton’s, so the weight is 30.1756 Newtons.

Initial thrust = 538.7 Newtons. Divide by weight of rocket 538.7/30.1756, so thrust to weight is 17.8522, if the parachute was included. Lower if the chute weight hasn’t been included yet.

#### RocketTree

##### Well-Known Member
I was currently trying to figure this out as well...

CTI G145 has initial peak thrust of 78lb, the rocket weighs 1.5lb, so for that brief moment it would experience 52g and have a 52:1 power to weight ratio?

Is that correct, or am I missing something there?

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#### mikec

##### Well-Known Member
If the thrust exactly equaled the weight, the rocket would just sit on the pad. So the peak acceleration in g's is the TTW ratio minus 1.

#### Ralph M Bohm

##### Well-Known Member
Okay, I think my calculations are well within the ballpark. The mass is "wet" with everything on the launchpad ready to go.

#### Ralph M Bohm

##### Well-Known Member
In determining Thrust-to-Weight ratio, isn't it done by taking the total impulse, divide it by the burn time and then, with that number, do the formula here:
Thrust-to-Weight Ratio (online)

Example:
The
Aerotech H100W-14A
has a total impulse of 234 N-sec
Divide is by the 2.3 second of burn time, you get an average of 101.74
Take 101.74 and plug it into
Thrust-to-Weight Ratio
Add in the total "wet" mass of the vehicle, say, 5.5lbs, to get:

4.14 T/W

Yes?

#### mikec

##### Well-Known Member
The initial thrust of most motors is substantially higher than the average thrust computed the way you describe, and that's what people are using.

You're certainly free to use the average thrust, but doing so may be very conservative depending on the motor's thrust curve. The point of computing TTW ratio is to make sure the rocket is stable when it leaves the rail, and a specific number is just a rule of thumb.